A conventional method of manufacturing a diffraction grating using a two-beam interference method is mainly employed in a DFB (Distributed FeedBack) laser to be described later.
The DFB laser uses a diffraction grating (periodical projection structure) formed in a waveguide as a reflection mechanism for a laser beam. The DFB laser oscillates in a single mode at or near the Bragg wavelength defined by the period of the diffraction grating. In addition, when the DFB laser is to be modulated at a high speed, since it is operated in the single mode, it is expected as a light source of a long-distance large-capacity optical communication system using optical fibers.
When a DFB laser having a waveguide on which a uniform diffraction grating is formed has both end faces having a small reflectance, since the DFB laser easily oscillates at two wavelengths one of which is shorter than the Bragg wavelength and the other of which is longer than the Bragg wavelength, the DFB laser cannot easily oscillate in a single longitudinal mode. Therefore, there is proposed a .lambda./4-shifted DFB laser which oscillates at a single mode such that the phase of a diffraction grating is shifted by .lambda./4 (a phase amount of .pi./2) in the central portion of a laser resonator. In addition, there is proposed a DFB laser which oscillates at a wavelength equal to the Bragg wavelength by a two-beam interference method using positive and negative photoresists (this DFB laser is described in, e.g., Electronics Letters Vol. 20, NO 24, 1984, PP 1,008-1,010). In specific element performance, it is reported that a high production yield can be obtained by using a .lambda./8-shifted diffraction grating having a shift amount half that of the .lambda./4-shifted diffraction grating (this is described in, e.g., the Institute of Electronics and Communication Engineers of Japan, Technical Report OQE86-150).
FIGS. 1A to 1F show a method of manufacturing a diffraction grating using positive and negative photoresists.
As shown in FIG. 1A, a negative photoresist 22 is formed on an InP substrate 21. An intermediate layer 23 is formed on the negative photoresist 22. A positive photoresist 24 is formed on the intermediate layer 23. As shown in FIG. 1B, the positive photoresist 24 is patterned. Thereafter, the intermediate layer 23 and the negative photoresist 22 are etched using the positive photoresist 24 as a mask. As shown in FIG. 1C, the intermediate layer 23 and the positive photoresist 24 are removed. Thereafter, a positive photoresist 25 is formed on the entire surface of the resultant structure. A two-beam interference exposure operation is performed using first and second laser beams 26A and 26B. As shown in FIG. 1D, the positive photoresist 25 is developed. As shown in FIG. 1E, after the InP substrate 21 is etched using a hydrogen bromide-based etchant, the positive photoresist 25 is removed. A positive photoresist (protection film) 27 is formed on only a portion where the InP substrate 21 is etched. Thereafter, the negative photoresist 22 is developed. As shown in FIG. 1F, the InP substrate 21 is etched using a hydrogen bromide-based etchant, and the negative and positive photoresists 22 and 27 are removed, thereby obtaining a phase-shifted diffraction grating.
In the method using the negative and positive photoresists, the negative and positive photoresists are simultaneously used. For this reason, the thicknesses of the photoresists and an exposure time of two-beam interference cannot easily be adjusted. The shapes of diffraction gratings formed in the regions of the negative and positive photoresists may be different from each other. In addition, when a shift amount of the phase of the diffraction grating is adjusted, the shift amount cannot help being set in an amount of .lambda./4 (i.e., a phase amount of .pi./2).
As a method of obtaining an arbitrary shift amount, there is a manufacturing method using a glass mask, e.g., reported in the Technical Report OQE85-60, the Institute of Electronics and Communication Engineers of Japan. In addition, there is also a manufacturing method using a phase shift film (1986, the Japan Society of Applied Physics, lecture No. 29P-T-8).
FIGS. 2A to 2D show a manufacturing method using a phase shift film.
As shown in FIG. 2A, a negative photoresist 22 is formed on an InP substrate 21. An intermediate layer 23 is formed on the negative photoresist 22. A phase shift negative resist film 28 is formed on the intermediate layer 23. Thereafter, the phase shift negative resist film 28 is patterned. A two-beam interference exposing operation is performed using first and second laser beams 26A and 26B. As shown in FIG. 2B, the intermediate layer 23 is removed from a portion which is not covered with the phase shift negative resist film 28. Thereafter, the negative photoresist 22 on this portion is developed. As shown in FIG. 2C, the InP substrate 21 is etched using a hydrogen bromide-based etchant. The negative photoresist 22 is removed from a portion which is not covered with the phase shift negative photoresist 28. In addition, the intermediate layer 23 and the phase shift negative resist film 28 are removed. Thereafter, a positive photoresist (protection film) 27 is formed on only a portion where the InP substrate 21 is etched, and the negative photoresist 22 is developed. As shown in FIG. 2D, the InP substrate 21 is etched using a hydrogen bromide-based etchant, and the negative and positive photoresists 22 and 27 are removed, thereby obtaining a phase-shifted diffraction grating.
In the manufacturing method using the phase shift film, when a photoresist used as the phase shift film is used as a phase shift medium, the photoresist has poor optical accuracy, since the film thickness of the photoresist is decreased during the development. In addition, a photoresist used as the phase shift medium naturally absorbs an ultraviolet beam. For this reason, optimal exposure times are disadvantageously different from each other in the region of the phase shift film and other regions. The method using a glass mask has the following drawbacks. That is, a process of manufacturing the glass mask is complicated, and an undesired multi reflection cannot easily controlled such that the glass mask is brought into tight contact with an object to be etched during the two-beam interference exposing operation.
In the manufacturing method using positive and negative photoresists or a phase shift film, a phase-shifted diffraction grating is manufactured by two etching processes. In the two etching processes, in order to prevent loss of a diffraction grating after the first etching operation, the diffraction grating is generally protected by coating a photoresist. However, a border between a region protected by the photoresist and a region etched in the second etching process cannot easily be made clear.
As described above, according to the conventional manufacturing method, when a phase-shifted diffraction grating is to be manufactured, the phase-shifted diffraction grating is transferred to a semiconductor substrate by two etching processes. For this reason, this manufacturing method has a unique drawback, and a phase-shifted portion disadvantageously has a step near a border between a region protected by a photoresist and a region etched by the second etching process.
The present invention has been made to solve the above drawbacks, and has as its object to provide a method of manufacturing a diffraction grating in which a uniform diffraction grating having an arbitrary phase shift amount can be formed by one etching process.